In the field of inorganic mass spectrometry, a constant necessity exists for yet even higher sensitivity, faster speed of measurement and greater precision, the latter e.g. for isotope ratio measurements or the use of isotope dilution analysis. ICP-MS, using an inductively coupled Argon plasma (“ICP”) at atmospheric pressure as the ionization source, has become the de-facto standard for inorganic mass spectrometry aiming at (trace and ultra-trace) concentration measurements. GD-MS, using a (typically Ar) glow discharge (DC or RF) at reduced pressure allows for sensitive direct concentration analysis of solid materials and thin layers, without digestion steps that might be required for (liquid sample based) ICP-MS, whereas TIMS, thermal ionization mass spectrometry, is mainly used for precise isotope ratio determinations, mainly in the geological field.
For ICP-MS, the mainstay (90% of the ICP-MS units shipped in 2005, Source: Global Assessment Report 9th Edition “The Laboratory Analytical and Life Science Instrument Industry 2006-2010”, SDI Los Angeles, September 2006) of commercially available instruments still employ a quadrupole mass filter as analyzer, a truly sequential (‘mass filtering’) device. Additionally, “high mass resolution” variants, employing a scanning sector field mass analyzer, e.g. of the (reverse) Nier-Johnson design, are known, which, using a single detector, again are sequential in nature. Only a very small amount of “semi-simultaneous” inorganic mass spectrometers exist commercially, either employing several (normally <10) detectors that can be moved in a focal plane, or using a time-of-flight mass analyzer. Up to now, no fully simultaneous (=allowing to simultaneously capture the full inorganic mass spectrum in one or only few (≦3) measurements) are commercially available, very much contrary to the situation in atomic emission spectrometry, where sequential spectrometers have mainly been replaced by simultaneous full-spectrum instruments.
Besides concentration information, also ICP-MS and GD-MS allow for the determination of isotope ratios of sample constituents. However, for precise isotope ratio determination, present day instrumentation in the field of inorganic mass spectrometry for a large part is limited by its sequential nature, resulting from the technologies employed. Caused by the unavoidable ion source fluctuations in both time and space, known e.g. as flicker noise for ICP-MS, the achievable precision for isotope ratio determinations is fundamentally limited for a sequential device, such as a quadrupole mass filter or a scanning sector field mass analyzer.
Apart from a more precise determination of isotope ratios, also other reasons make a (fully) simultaneous inorganic mass spectrometer desirable. For instance, the “multiplex” or “Fellgett's Advantage” dictates that for a continuous multi-channel signal and for a given total integration time a simultaneous measurement of all channels results in a better signal-to-noise ratio than a sequential channel after channel measurement of the same (total) integration time. In inorganic mass spectrometry, this advantage translates into shorter measurement times to reach the same signal to background ratio, especially if many masses (isotopes) are to be measured (as in typical environmental applications). Shorter measurement times per sample increase the sample throughput, with the positive effects of less sample consumption (and the possibility of using smaller sample volumes), less energy and media consumption and finally less waste production.
Apart from higher sample throughput, in inorganic mass spectrometry, faster measurement times, independent of the number of channels (=isotopes) sampled, are of great interest for all methods generating transient signals, signals that change as a function of time. Among such methods are, for instance, the on-line coupling to inorganic mass spectrometry instruments (“hyphenation”) of separation systems like gas- or liquid phase chromatography, of direct solid sample introduction systems like electro-thermal vaporization, spark or laser ablation, or of flow injection systems. All transient signal generating methods require short measurement times per data point in time, to avoid the so-called “peak skewing”, caused by missing the maximum signal of a given transient signal resulting from slow scanning, as found in sequential devices (transient signal artifacts resulting from insufficient sampling frequency). Thus, for sequential devices, the amount of channels that can be measured reliably with (fast) transient signals is strongly limited, compared to a (fully) simultaneous device, able to measure “all” channels reliably as a function of time. With hyphenation methods gaining more and more interest in inorganic mass spectrometry, it is obvious that a (fully) simultaneous device is instrumental in achieving better data quality with transient measurements.
The same benefit of shorter measurement times applies for GD-MS e.g. in the case of thin layers analysis. Here, the amount of constituents that can be determined simultaneously with a given depth resolution is limited by the scan speed of a sequential device. In contrast to that, a (fully) simultaneous instrument allows for a constant depth resolution, independent of the number of constituents desired to be analyzed.
It is thus clear after the aforementioned advantages of such system that a clear need exists for an inorganic mass spectrometer, capable of measuring simultaneously the full inorganic mass range (typically Li6 to U238), in one or very few (≦3) individual measurements. It is also obvious that this need currently commercially is neither fulfilled by sequential instruments based on the quadrupole mass filter, nor the existing so-called “multi-collector” instruments, having only a very limited number (<10) of simultaneous channels. Apart from the mentioned, also TOF (time-of-flight) mass analyzer based instruments for inorganic mass spectrometry exist commercially. Although TOF based instruments allow for a simultaneous spectrum capture, their pulsed sampling nature resulting from the need of waiting for the ion pulse sampled to fully travel through the drift tube of the analyzer results in a limited sample rate that does not utilize the cw ion beam originating from the ion source, e.g. the atmospheric Ar plasma. Compared to a truly fully simultaneous inorganic mass spectrometer, the TOF analyzer based system exhibit worse signal-to-noise ratios for a given total integration time, depending on their sampling rate and duty cycle, as expected from sampling theory. Again, the advantages of a fully simultaneous system over TOF-based systems are obvious.
One goal of the claimed invention may thus be to remedy this unsatisfying situation by creating a fully simultaneous inorganic mass spectrometer, allowing to measure the full inorganic mass range (typically Li6 to U238) in a single or very few (≦3) measurements. Apart from providing means for a simultaneous measurement over a large mass range, also sufficient signal dynamic range and resolution of adjacent mass signals may be provided, as common in inorganic mass spectrometry; see, e.g., Montaser, Akbar, ed., Inductively Coupled Plasma Mass Spectrometry, Wiley-VCH (1998) for a discussion of typical specifications for commercially available ICP-MS.